Activated carbon treated by carbon dioxide for the stabilization of treated water pH

ABSTRACT

Treatment of a wet activated carbon with carbon dioxide or with carbon dioxide followed by air results in a carbon having a reduced contact pH. The activated carbon is characterized by a modified contact pH less than about 9.0 and typically between about 7.3 and 9.0. Use of this carbon in a water treatment system eliminates the excessive effluent water pH rise which commonly occurs with activated carbon.

FIELD OF THE INVENTION

The present invention relates to a process for producing an activatedcarbon for the stabilization of pH in water treatment processes. Moreparticularly, this invention relates to the production of asurface-modified, activated carbon having a reduced contact pH usingcarbon dioxide or using carbon dioxide with a subsequent air treatment.

BACKGROUND OF THE INVENTION

In the water treatment industry, whether municipal, industrial, orremediation, the continued use of standard carbon products causes theeffluent pH to increase relative to the influent water pH and often theeffluent water pH exceeds 9. This pH excursion occurs with virgin andreactivated carbon and is independent of the raw material. For example,pH excursions have been identified or associated with activated carbonsthat are made from bituminous coal, sub-bituminous coal, peat, wood, andcoconut. The use of carbon having a reduced contact pH to stabilize thepH in water treatment has become available to assist in overcoming theseproblems, see, e.g., U.S. Pat. Nos. 5,368,738, 5,368,739 and 5,466,378.

Work with these new modified activated carbons has shown that the carbonsurface oxidizes at high temperatures with oxygen or air, at ambienttemperature with oxygen or air, or with other oxidants such ashypochlorite, nitric acid, and ozone. With this oxidation, the surfaceof the activated carbon is changed such that the affinity or adsorptioncapacity for anions such as sulfate is reduced. The adsorption of theseanions has been associated with pH excursions. Problems caused by the pHexcursions include reduced throughput due to recycle of the high pHwater, down time in operation of dialysis systems as the pH is broughtinto control, wasting water that is high in pH and does not meet therequirements of reverse osmosis systems for high purity water, andmonetary fines for exceeding permitted pH levels in wastewaterdischarge. Historically, the high pH water is alleviated throughexcessive back washing of the carbon or neutralization of the waterthrough the use of strong acids such as hydrochloric acid or sulfuricacid or a weaker acid such as carbonic acid. These processes are bothtime consuming and expensive.

Specific characteristics of pH excursions have been described in U.S.Pat. Nos. 5,368,738, 5,368,739 and 5,466,378. In summary, the patentsteach that a pH increase in the effluent water from an activated carbonwater treatment system is triggered by the presence of anions such aschloride, nitrate, sulfate which occur naturally in water. The art alsoteaches that activated carbon characterized by a contact pH about 8.5 to9.0 will exhibit pH excursions with water containing anions such asthose stated above. Furthermore, the higher the carbon contact pH thegreater the extent of the excursion. U.S. Pat. Nos. 5,368,738 and5,466,378 teach that the contact pH of the carbon can be reduced byoxidation at elevated temperature. U.S. Pat. No. 5,368,739 teaches thatthe carbon contact pH can be reduced by oxidation at or near ambienttemperature.

Accordingly, it is an object of the present invention to provide aprocess for producing a modified activated carbon having a contact pHbetween 6.0 and 9.0. It is also an object of the invention to provide aprocess for producing a modified activated carbon to eliminate processrelated problems that are associated with elevated temperatureoxidation, such as reduced carbon yield, and to overcome the longtreatment times or high gas volumes that are associated with oxidationat or near ambient temperature. It is a further object of the inventionto provide a process for producing a modified activated carbon which ishighly efficient and cost effective for use in the prevention of pHexcursions in water treatment systems.

SUMMARY OF THE INVENTION

The present invention provides a method using carbon dioxide without thepresence of oxygen for oxidation to produce a modified activated carbonespecially useful in water treatment systems. The process comprisescontacting a wetted activated carbon with carbon dioxide. The carbondioxide reacts with the carbon surface and neutralizes the surface sitesthat normally remove anions during water treatment causing the water pHto increase. Preferably, the wetted activated carbon is contacted withthe carbon dioxide at or near ambient temperatures. Alternatively, thewetted carbon is contacted with the carbon dioxide and then air. Theresulting modified carbon is highly effective at minimizing watertreatment system pH fluctuation.

The treated activated carbon of the present invention is characterizedby a reduced contact pH. The contact pH is measured after contactingactivated carbon with a sodium sulfate solution for 30 minutes, asdescribed hereinafter in the Analytical section. In particular, thetreated activated carbon is characterized by a contact pH less thanabout 9.0. The activated carbon can then be used inadsorption/filtration systems for the purification of water. The pH ofthe water from such a water treatment system is then controlled tolevels that are acceptable to the user. Other features of the inventionwill become apparent from a perusal of the presently preferredembodiments of the invention taken in connection with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 graphically illustrates the pH profile that occurs with thevolume of treated water for virgin carbon and also the carbon of thepresent invention. The effluent water pH profiles in FIG. 1 are for acommercially available activated carbon, an activated carbon treatedwith carbon dioxide by the method of the present invention and also foran activated carbon treated with carbon dioxide and then air by themethod of the present invention.

FIG. 2 graphically illustrates that the performance of the carbons ofthe present invention are not dependent upon the influent water pH.

PRESENTLY PREFERRED EMBODIMENTS

Experimental

Testing was performed with one inch inside diameter, Pyrex glasscolumns. The columns used for the carbon dioxide or the carbondioxide/air contacting were 12 inches long and contained either 50 cc or100 cc of activated carbon. Prior to contacting the carbon with carbondioxide, the carbon was wetted by soaking in tap water in a beaker forabout 16 hours. The wet carbon was then transferred to the 12 inchcolumn, the water drained from the column, and the gas flow began. Thetotal gas flow was measured with the use of a calibrated rotameter andstop watch. After completing the gas contact in those tests using 100 ccof activated carbon, one half of the treated carbon was tested forcontact pH (Calgon Carbon Corporation Test Method TM-70) and the otherhalf was transferred to a six-inch long column to measure the effluentwater pH profile. The columns used for developing the pH profile weresix inches long and contained 50 cc of activated carbon. The waterpassed through the six-inch column was Robinson Township MunicipalAuthority tap water. The flow rate was 10 cc/min for an empty bedcontact time of 5 minutes. The pH of the column effluent was monitoredcontinuously using an in line pH electrode. In those tests using 50 ccof activated carbon, only the contact pH was measured.

Analytical

The contact pH of the activated carbon, prior to and following treatmentwith carbon dioxide or carbon dioxide and then air, was determined bycontacting 50 cc of the activated carbon with 100 cc of a sodium sulfatesolution for thirty minutes. The sodium sulfate solution was preparedfrom water obtained from a Milli-Q Plus water treatment system(Millipore Corp. Bedford, Mass.) and Fisher Certified ACS grade sodiumsulfate (Fisher Scientific Corp. Pittsburgh, Pa.) such that the sulfateconcentration in the water was 80 mg/L. The sulfate solution was addedto a beaker containing the carbon and gently stirred for 30 minutes witha magnetic stirrer. At the end of this 30 minute time period, stirringwas stopped and the pH of the solution was measured.

Activated Carbons

Tests to illustrate the present invention were performed using severaldifferent types of activated carbon. These activated carbon typesincluded both virgin and reactivated activated carbon representingseveral different mesh sizes. The carbons selected were typical of thoseused to treat air and liquid streams. The carbons evaluated included BPL4×6, F300 8×30, React AW 8×40, F400 12×40, and PCB 20×50 (Manufacturedby Calgon Carbon Corporation, Pittsburgh, Pa.). All of the productstested were bituminous coal based carbons with the exception of PCBwhich is a coconut based carbon.

Carbon Processing According to the Present Invention

Carbons were treated with carbon dioxide (CO₂) or CO₂ and then air in aone-inch ID by 12 inch long Pyrex glass column as described in theprevious Experimental section. For the treatments with CO₂, the CO₂ gasvolume ranged from one bed volume (approximately 100 cc) to 240 bedvolumes. A bed volume is the gas volume equivalent to the volume ofactivated carbon. The volume of activated carbon is the weight ofactivated carbon divided by the apparent density of the activatedcarbon. The carbon dioxide flow rate was typically set at 100 cc/min or1 bed volume each minute so that the total treatment time ranged fromone minute to four hours. For the treatments with carbon dioxide andthen air, the CO₂ flow rate was set at 100 cc/min and the treatment timewith carbon dioxide was set at five minutes. This treatment was followedby injecting air at a flow rate of 100 cc/min (1 bed volume eachminute). The total air treatment time ranged from 5 minutes to 60minutes.

Table 1 shows that treatment of a wet F400 activated carbon with carbondioxide reduces the carbon contact pH to acceptable levels (less thanabout 8.5 to 9.0) with as little as 0.1 liters (or one bed volume) ofgas for 0.1 liters of activated carbon. This reduction in contact pH issufficient to prevent the pH excursion that occurs with untreated F400carbon, as exhibited in FIG. 1 using Robinson Township tap water as theinfluent water. The data in Table 1 also show that the treatment of thecarbon with CO₂ can be conducted either upflow or downflow as verysimilar carbon contact pH measurements are obtained.

Table 1 demonstrates that treatment of wetted activated carbon withcarbon dioxide reduces the contact pH of that carbon to levels that aregenerally considered to be acceptable. In those situations where furtherpH reduction in the water treatment system effluent is desired, the CO₂treatment can be followed by treatment with air, to result in additionalreduction in the water treatment system effluent pH. Air is used toillustrate the effect of a gas containing oxygen. As shown in FIG. 1,when carbon treated with carbon dioxide alone is first brought on line,the initial water effluent pH is about 6.2 and a maximum effluent pH of8.5 can typically be expected. However, by following the carbon dioxidewith air, the maximum effluent pH decreases, as depicted in FIG. 1. Thisprocess can be utilized for systems that require a water pH closer toneutral. Table 2 also shows that the modified contact pH of the carbontreated first with carbon dioxide and then air remains in the regionthat is classified as a pH stable carbon, i.e., below about 8.5 to 9.0for extended air volume treatment.

The carbon dioxide treatment or the carbon dioxide and air treatment toreduce carbon contact pH for activated carbon can be applied to manytypes of activated carbon. As shown in Table 3, the carbon dioxide orthe carbon dioxide/air treatment can be applied to carbon of variousmesh size. Also, the processes can be applied to reactivated carbon andcan result in a contacty pH of 7.3. Finally, the process can be appliedto coconut base carbon (PCB 20×50). This is an improvement over theprior art which teaches ambient temperature oxidation (U.S. Pat. No.5,368,739) as such oxidation produced a contact pH of 9.7 for coconutbased carbon.

Dilute carbon dioxide can also be used in the present invention. Table 4shows the percentage of carbon dioxide in the contacting gas is notcritical. Rather, it is the volume of carbon dioxide that contacts theactivated carbon. The volume of carbon dioxide required for producing anactivated carbon with a contact pH at or below about 9.0 requiresbetween one and two bed volumes of carbon dioxide irrespective of thepercentage of carbon dioxide in the treating gas. Thus it is possible toutilize off gas from other processes with lower carbon dioxide partialpressure. For example combustion gas or flue gas could be used tocontact wet activated carbon to produce the carbon of the presentinvention.

The present invention expresses gas to carbon ratios as volume to volumeratios. It is equally appropriate to convert these values using wellknown conversion factors and equations and express the gas to carbonratios as volume to mass, mass to volume, or mass to mass relationships.

                  TABLE 1                                                         ______________________________________                                        Effect of Carbon Dioxide Volume on Activated Carbon pH                        Gas/Carbon Total Gas Volume                                                                            Direction of                                                                             Carbon                                    Contact Time, min                                                                        liters        Gas Flow   pH                                        ______________________________________                                        0          0             None       10.6-10.7                                 240        24.0          Downflow   7.8                                       60         6.0           Downflow   7.9                                       15         1.5           Downflow   7.8                                                                Upflow     7.6                                       10         1.0           Downflow   7.9                                       5          0.5           Downflow   7.9-8.0                                   3          0.3           Downflow   7.8                                       1          0.1           Downflow   8.5                                       ______________________________________                                         Activated Carbon  100 cc Filtrasorb 400 (F400)                                Carbon dioxide flow rate  0.1 L/min                                      

                  TABLE 2                                                         ______________________________________                                        Effect of Carbon Dioxide Followed by Air on Activated Carbon pH                       Time    Volume    Direction of                                                                          Carbon                                      Gas     Minutes Liters    Gas flow                                                                              Contact pH                                  ______________________________________                                        Air      5      0.5       Downflow                                                                              7.6-7.8                                     Air     15      1.5       Upflow  8.3                                         Air     60      6.0       Downflow                                                                              8.5                                         ______________________________________                                         Activated carbon  100 cc of Filtrasorb 400 (F400)                             Carbon dioxide flow rate  0.1 L/min for 5 minutes in all cases           

                  TABLE 3                                                         ______________________________________                                        Effect of Activated Carbon Particle Size                                                              Volume  Carbon                                        Carbon      Gas         Liters  Contact pH                                    ______________________________________                                        BPL 4x6     CO2/Air     0.5/0.5 8.7                                           BPL 4x6     C02         0.5     8.8                                           F300 8x30   C02         0.5     7.7                                           React AW 8x40                                                                             C02         0.3     7.3                                                                   0.5     7.3                                           PCB 20x50   C02         0.5     8.2                                           ______________________________________                                         Gas flow rate  0.1 L/min                                                 

                  TABLE 4                                                         ______________________________________                                        Effect of Dilute Carbon Dioxide on Activated Carbon pH                        Percent   Total Gas  Carbon Dioxide                                                                             Carbon                                      Carbon Dioxide                                                                          Volume, cc Volume, cc   Contact pH                                  ______________________________________                                        10        500        50           9.2                                         10        6000       600          8.4                                         20        200        40           9.6                                         20        400        80           9.0                                         20        800        160          8.4                                         50        100        50           9.5                                         50        200        100          8.7                                         50        500        250          8.3                                         ______________________________________                                         Activated Carbon  50 cc of Flltrasorb 400                                     Gas Flow Rate  0.1 L/min                                                      Gas Composition  Carbon dioxide and Nitrogen                             

While presently preferred embodiments of the invention have beendescribed in particularity, the invention may be otherwise embodiedwithin the scope of the appended claims.

What is claimed is:
 1. A process for producing an activated carbonhaving a contact pH below about 9.0 comprising the steps of:a. wettingan activated carbon with water to produce a wet activated carbon and b.contacting said wet activated carbon with a gas comprised substantiallyof carbon dioxide wherein the amount of carbon dioxide contacted withsaid wet activated carbon is greater than about one bed volume of saidcarbon dioxide.
 2. A process as set forth in claim 1 wherein said wetactivated carbon is contacted with air after said contacting with carbondioxide.